How to combine Cycles and Blender render god-rays?
If you set Alpha to Transparent in Render Settings > Shading for the BI scene:



The render will have an alpha channel: (Note that you have to enable RGBA in Render Settings > Output to be able to save it with the alpha channel)



(I made the spot lamp red so it would be visible here)

You can then use the Alpha Over node to mix this with a Cycles render:



You could also use the alpha channel as a Factor: